(2014) – the History and Dynamics of a Welded Pyroclastic

Bull Volcanol (2014) 76:811 DOI 10.1007/s00445-014-0811-0

RESEARCH ARTICLE

The history and dynamics of a welded pyroclastic dam and its failure

Graham D. M. Andrews & James K. Russell & Martin L. Stewart

Received: 17 July 2013 /Accepted: 15 February 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The 2,360 BP eruption of Mount Meager, British approximately 8 h and eroded a 2.5-km long canyon into the Columbia began as an explosive, dacitic sub-Plinian eruption still-hot dam core before returning to background flow rates. that waned rapidly to a sustained period of Vulcanian, eruption-triggered dome collapse events producing volumi- Keywords Block and ash flows . Volcanic dams . Welding . nous block and ash flow (BAF) deposits. The earliest BAF Dam failure . Lahar deposits accumulated rapidly enough immediately downslope of the vent to retain heat and weld; using the deposit as a paleoviscometer determines an effective viscosity of 109– Introduction 1010 Pa s during welding. This prolific production of hot lava and block and ash flows, in a steep mountainous terrain, Pyroclastic density currents and volcanic mudflows (“lahars”) created a ∼110 m high, largely impermeable dam capped by account for 40 to 50 % of all fatalities related to volcanic permeable, non-welded BAF deposits and unconsolidated eruptions (Auker et al. 2013) including the greatest volcanic avalanche deposits that blocked the flow of the Lillooet disasters of the twentieth century (Mont Pelée, Martinique, River and created a temporary lake. The welded pyroclastic 1902; Nevada del Ruiz, Colombia, 1985). Although pyroclas- dam was compromised and overtopped at least once before tic density currents have the potential to cause tens of thou- the peak dam height was reached. Renewed eruption caused sands of fatalities as infrequent events, lahars cause tens to buildup of the dam to a maximum of 780 m above sea level hundreds of fatalities annually (Auker et al. 2013). Lahars are (asl) and grew the temporary lake to an elevation of 740 m asl particularly challenging hazards to plan for, and mitigate and a minimum volume of 0.55 km3. The rise of lake level led against, because they can occur during and for several years to catastrophic failure of the top of the dam, generating an after an eruption (Manville and Cronin 2007). Many lahars outburst flood that carved a canyon through most of the dam result from the post-eruption failure of temporary lava- and resulted in a voluminous lahar that is traced at least 65 km generated dams (e.g., Fenton et al. 2006) and volcaniclastic- downstream. Based on current flow rates of the Lillooet River, generated dams (Capra 2007), and the associated catastrophic the lake would have overtopped the final dam at a minimum of release of the impounded lake. Examples include the 1982 39–65 days after its formation. The peak deluge lasted eruption of El Chichón, Mexico (Macías et al. 2004); Nevado de Colima, Mexico (Capra and Macías 2002); Ruapehu 2007, New Zealand (Manville and Cronin 2007); and Numazawa, Editorial responsibility: V. Manville Japan (Kataoka et al. 2008). In this paper, we describe and interpret the proximal and * G. D. M. Andrews ( ) medial volcanic deposits associated with damming of the Department of Geosciences, California State University Bakersfield, Bakersfield, CA 93311, USA Lillooet River during the 2,360 BP eruption of Mount e-mail: [email protected] Meager, B.C. We use field observations to reconstruct the : : temporal and spatial building of the dam, including the height G. D. M. Andrews J. K. Russell M. L. Stewart and duration of the dam, the size of the lake, and the down- Volcanology & Petrology Laboratory, Department of Earth & Ocean Sciences, University of British Columbia, Vancouver, BCV6T 1Z4, stream extent of the outburst debris flow deposits. Our work Canada provides semiquantitative estimates on the rates, volumes, and 811, Page 2 of 16 Bull Volcanol (2014) 76:811 timescales of dam-building, lake-filling, and dam failure. This Fig. 2 a Map of the 2,360 BP Pebble Creek Formation (modified from is the first described occurrence of a partially welded pyro- Hickson et al. 1999). b Non-exaggerated oblique south-southwest view of clastic dam and a rare case where field evidence can elucidate the Mount Meager volcanic complex (MMVC) and Lillooet River valley (adapted from Google EarthTM). c Panorama and dimensions of the the timescales of events preceding and attending the failure of canyon incised into unit A; viewed downstream (east-southeast) from a volcanic dam (cf. Capra 2007). Keyhole Falls

Geological setting The southern Coast Mountains of British Columbia (Fig. 1) have been characterized by rapid uplift rates and enhanced Mount Meager volcanic complex glaciofluvial erosion rates over the past 4 Ma (Clague et al. 1982;Andrewsetal.2012). The MMVC is now highly The 2,360 BP eruption of the Mount Meager volcanic com- dissected and its base is perched at 1,100–1,200 m above plex (MMVC), Cascades arc, British Columbia (Fig. 1; Read sea level (asl), more than 500 m above the present-day erosion 1977) is the youngest explosive volcanic event in Canada surface marked by the base of the adjacent Lillooet River (Hickson et al. 1999). The eruptive center poses a significant valley (400–600 m asl; Fig. 2). The 2,360 BP eruption vent risk to local communities from both volcanic eruptions and is situated immediately above the narrow and steep-sided, non-volcanic mass-wasting events (Simpson et al. 2006; glaciated valley hosting the Lillooet River, and, as a conse- Friele et al. 2008). quence, this portion of the Lillooet River valley is partially filled with a thick (0–180 m) accumulation of proximal pyroclastic deposits.

MT. MEAGER Physiography of Upper Lillooet River valley

The Upper Lillooet River valley (Fig. 2a) is a steep-sided, narrow, glaciated valley situated immediately north of the British ∼ Columbia MMVC and extending 16 km upstream towards the Lillooet icefield (Fig. 2b). The Lillooet drainage was sculpted by Late EXPLORER inset Pleistocene glaciation; however, the post-glacial physiography PLATE of the Lillooet River is significantly different along a ∼2.5 km stretch immediately below the MMVC and southeast of Keyhole Falls (Fig. 2a), where it has been infilled by deposits Washington of the Pebble Creek Formation (PCF: Hickson et al. 1999). At Keyhole Falls, two morphologically distinct canyons JUAN DE FUCA are carved into PCF deposits—(1) a <150 m deep, ∼300 m PLATE wide, horseshoe-shaped canyon (Fig. 2c) carved through the entire thickness of the PCF to the basement and (2) a 60 m deep, <15 m wide slot canyon carved into welded BAF MOUNT MEAGER Oregon VOLCANIC deposits (Fig. 3a). The larger horseshoe-shaped canyon is COMPLEX straight, rough sided, rubble strewn, and extends approximate- Lillooet ly 2.5 km downstream of Keyhole Falls. The smaller slot River canyon is short (<1 km in length), sinuous, and has water- Pemberton polished vertical surfaces and extends upstream of Keyhole Falls. Mt. Cayley The slot canyon is typical of perennial water-carved slot Whistler canyons (e.g., Carter and Anderson 2006) resulting from California steady long-term stream flow and abrasion. However, the scale, shape, and textural morphology of the larger canyon Mt. Garibaldi (Fig. 2c) are atypical of fluvially carved canyons. The width Squamish and depth of the canyon are at least an order of magnitude 0 20 km 0 200 km larger than can be supported by the present-day fluvial discharge. Such misfit stream–canyon relationships commonly Fig. 1 Map of the Cascade volcanic arc (triangles) of western North America, in southern British Columbia, Canada, and location of Mount indicate rapid and sudden alteration of local drainage systems Meager (inset) within the Garibaldi volcanic belt (e.g., local tectonism, enhanced glacial erosion, or outburst Bull Volcanol (2014) 76:811 Page 3 of 16, 811

a 2200

2000

1800

1600 Salal Creek 800 Lillooet 1200 1400

River 1000 N

800 Keyhole Falls

1000

1200

1400 1600

PCF

inferred vent

1000 2000

800 2200 Plinth Peak

2400 600

Meager Creek

400

b Lillooet Icefield

MMVC

inferred vent

Lillooet River Meager Creek

Lillooet River Keyhole Falls

Salal Creek N

non-welded block & ash flow and sedimentary deposits

90 m

welded block & ash flow deposit ~300 m 811, Page 4 of 16 Bull Volcanol (2014) 76:811

a simplified lithostratigraphy at Keyhole Falls 780 non-welded block and ash flow deposits unit C

BAF, gravel, and rock avalanche 700 slot-canyon deposits unit B resistant 3C surface

welded block 3B and ash flow deposits unit A

600

m asl b c unit C welded BAF deposit

non-welded BAF unit B & rock avalanche deposits

welded BAF deposit sand and gravel

unit A

Fig. 3 a The pyroclastic dam at Keyhole Falls (looking northwest) and volcaniclastic succession (unit B) buried by a later non-welded BAF the Keyhole Falls slot canyon. The locations of photos b and c are shown. deposit (unit C); note the presence of alternating recessive (soft, non- b Close-up of irregularly fanning columnar joints in unit A in the canyon welded BAF, sand, gravel, and rock avalanche deposits) and cliff-forming wall at Keyhole Falls (courtesy of Paul Adams); the slot canyon is 60 m (hard, welded BAF deposits) horizons deep. c View of the northern canyon wall and the 45-m thick clastic and

floods). Other examples in volcanic settings include down- et al. 1996; Hickson et al. 1999). The intensity of the eruption stream of Lake Taupo (Manville et al. 1999;Manvilleand waned to a Vulcanian explosive phase (Michol et al. 2008) Wilson 2004) and Lake Tarawera in New Zealand (Hodgson producing a sequence of welded (early) and non-welded (late) and Nairn 2005;Manvilleetal.2007). BAF deposits (Fig. 2a). The estimated volumes of units within the PCF are given in Table 1. The Pebble Creek Formation The PCF deposits filling the Lillooet River valley have an asymmetric, bivergent, wedge-shaped cross-sectional profile The 2,360 BP eruption produced a >0.58 km3 (VEI 4) se- (Fig. 4) and, near Keyhole Falls, they comprise a 0–200 m quence of dacitic (SiO2 67–69 wt.%) pyroclastic deposits, a thick sequence of poorly sorted, non-welded, incipiently lava, and related volcaniclastic deposits that constitute the welded, and strongly welded BAF deposits with a total vol- 3 Pebble Creek Formation (Hickson et al. 1999; Table 1). The ume of 0.15 km (Table 1;Fig.3;unitsbx2 and bx3 of Hickson vent is situated at 1,400 m asl and approximately 800 m above et al. 1999). The thickest section is found immediately down- the present streambed of the Lillooet River (Fig. 2b). The slope of the inferred vent at Keyhole Falls (Figs. 3a and 4). At 2,360 BP eruption began with a sub-Plinian phase that gener- this location, two successions of BAF deposits (units A and C) ated pumice fall deposits and associated ignimbrites (Stasiuk are exposed, separated by a 0–45 m thick sequence of poorly Bull Volcanol (2014) 76:811 Page 5 of 16, 811 a ) D

E sorted volcaniclastic and epiclastic deposits (unit B) that in- 24 – weeks 2 2 31 – 2to

– – cludes a distinctive basal lens of moderately well-sorted, – ∼ (days) >14 t 4h coarse-grained sediment. ≤ Δ

Welded block and ash flow deposit—unit A

∼ 31 after Unit A (bx2 of Hickson et al. 1999)isa 110-m thick, vitric – resumption of dam growth 2ca. (days) 25 – breccia comprising angular, black obsidian blocks in a welded o N.A. Weeks 1 ∼ Unconstrained >25 t gray, vitric matrix; the unit is characterized by ubiquitous 1-m- wide columnar joints that extend from the base to the capping disconformity (Fig. 3a). It is exposed in the Lillooet River

) and maximum and minimum elevations ( canyon walls at Keyhole Falls (Figs. 2 and 3a) where the base D L ) of events associated with lithofacies t E H and top of the unit lie at 570 and 680 m asl, respectively, and Δ 160 m, 740 m asl 100 m, 680 m asl; ∼ and for 2 km downstream. Unit A is locally underlain by a non- — L —

H welded pumice fall deposit, non-welded ignimbrites, and : min. 100 m, 680 m asl >14 Days <170 m, 740 m asl :Lakedrained :max :min basement rocks (Fig. 4). Unit A is unconformably overlain — by a 0- to 45-m thick succession of volcaniclastic and clastic (max. depth 160 m) max (strandline and top of delta) ) and durations ( Lowering Growing Growing Lowering Salal Creek – o deposits (unit B) which are capped by 0 50 m of dominantly t non-welded BAF deposits (unit C; cf. bx3 of Hickson et al. 1999). Unit A is inferred to be a BAF deposit because it features: D E , contribution to thickness of dam ( (1) monomict juvenile clasts of vitric dacite (Fig. 3b), (2) 0m,570masl 740 m asl ≥ and

— angular clasts, (3) ash-rich matrix, (4) poor sorting, and (5) 110 m, 170 m, 740 m asl; 110 m, 680 m asl; D — H — — has an apron-shaped distribution (Fig. 2a). The intense

170 m, welding, a feature atypical of BAF deposits, probably results : min. 110 m, 680 m asl :max ≥ <170 m, 740 m asl :min :min — — from relatively high emplacement temperatures and rapid (erosion surface) 680 m asl; min max max accumulation within a narrow, steep, mountainous drainage Lowering Growing Lowering Growing N.A.system Top of lake at 740 m asl (Michol et al. 2008). Downstream of Keyhole Falls, the columnar joints are oriented radially, horizontally, or cha- otically with respect to exposed cliff faces (Fig. 3b); this umes, interpreted geologic event suggests that the canyon was eroded into the deposit while it mentation was still hot and above its glass transition temperature (Tg), but after welding (Hickson et al. 1999). losive frag ) taken at Salal Creek; and onset times relative to eruption ( L E Unit B and the erosion surface of dam and downstream

hofacies, estimated vol A resistant erosional surface (680 m asl; Fig. 4) defines the contact between the welded BAF deposit (unit A) and unit B; an overlying, 0- to 45-m thick, downstream-tapering wedge of ) and elevations ( L dam-overtopping, denudation, and sediment transport/deposition dome and lava debris torrent; canyon excavation of a conduit-filling plug dam

H clastic and volcaniclastic sediments (Fig. 3a). Columnar joints Sub-Plinian eruption columnCollapse of ash and pumice-laden jetsDensity currents from exp <5 m, 570 mWaxing <2 asl volcanic m, input 570 from m decrepitation asl of a Catastrophic failure 0 m, 580 m 0 asl m, 580 m asl 0 0 ca. 1 ca. 1 ation is unlikely to be many months or years in unit A are sharply truncated at the erosion surface and ) Interpretation Keyhole Falls 3 directly overlain by unconsolidated coarse clastic and 8

7 8 7 7 volcaniclastic materials (Fig. 3c). There is no gradational contact between welded and non-welded BAF deposit, as 5×10 1×10 negligible Competing (1) volcanic dam-building and (2) N.A. Lacustrine deposition upstream of volcaniclastic would be expected in a complete and continuous section through the upper non-welded parts of a welded pyroclastic deposit (c.f. Smith 1960). Unit B is largely inaccessible as it is only exposed in treacherous cliffs at Keyhole Falls immediately above the canyon sidewalls (Fig. 3a); therefore, all observations come Summary of the 2,360 BP Pebble Creek Formation lit from afar (e.g., through binoculars). This succession of coarse clastic and volcaniclastic deposits is highly asymmetric across flow (unit B) (unit A), 1-m thick beds welded block and ash flow deposits strandline No evidence of hiatus in volcanism, thus, dur Non-welded block and ash Non-welded ignimbritesWelded block and ash flow 1×10 Sediment lens with interbedded Lahar deposit 5×10 Volcaniclastic delta and Slot canyon N.A. Fluvial erosion Incised 60 m into dam remnant N.A. After dam failure 2,400 years Pumice fall 4.2×10 measured at Keyhole Falls; height of paleo-lake ( Ta b l e 1 Deposit/feature Volume (m a the canyon; the southwest canyon wall (i.e., below the inferred 811, Page 6 of 16 Bull Volcanol (2014) 76:811

c 780

& unit B B&A max elevation of lake 2360 BP DAM = 740 m asl AB C D E F 800 strandline 740 apex of delta G modern m asl sediments Lillooet River PALEO-SALAL LAKE 2x vertical exaggeration topographic 400 profile 700 2000 1000 010002000 3000 a b m

680 m asl unit bns 670 A 1300 m bns 850 m distance from Keyhole Falls (C)

upstream A unit B&A downstream

debris flow deposit

stratigraphic tie-line block & ash flow deposit (non-welded) (observed) 600 d

ation stratigraphic tie-line conglomerate, gravels lake / alluvial (inferred) deposits 580 unit B valley-flank event sand tree root 570 bns 570 unit A avalanche breccia deposit vitric clast 0 m unit A bns 2360 BP block & ash flow deposit (welded) columnar joints

540 Peeble Creek Form ignimbrite radial-jointed clast 900 m e valley-axis 2360 BP DAM pumice fall deposit bns base not exposed Wisconsinan unit A glaciation bedrock unconformity 500 two ignimbrites 1800 m N f A C B D OUTBURST E 460 bns FLOOD lava 2850 m F g

420 3600 m Fig. 4 Stratigraphic correlation diagram of the Pebble Creek Formation sections A–G. Cross-section (upper right inset) showing wedge-like including A–B deposits of the paleo-Salal Lake upstream of the dam, C–E geometry of the ≤220 m high dam. The heavy black line represents the the pyroclastic dam deposits, and F–G outburst flood deposits (lahar current level of the Lillooet River deposits) downstream of the dam. Inset lower left shows locations of

volcanic vent) is overlain by 20–30 m of deposits while the entire sequence was pyroclastic (i.e., unit bx3 of Hickson northeast side of the Lillooet Valley (i.e., across from the et al. 1999); however, unit C is distinguished here because inferred volcanic vent) hosts ∼45 m of the same material. the deposition of non-volcanic sediment on top of the erosion The sequence is thickest at Keyhole Falls and tapers down- surface indicates a break or significant reduction in pyroclastic stream and is absent 1 km downstream. input. A 0- to 10-m thick lens of sand and sub-rounded gravel We interpret the erosional surface to record an interval of directly overlies the erosion surface at the top of the welded fluvial erosion and denudation of the volcaniclastic dam down BAF deposit (unit A); it is overlain by a 5-m thick welded to its welded core (unit A), thus stripping-off the original BAF deposit (Fig. 3c). The sediment lens appears to be unconsolidated, non-welded upper portion of unit A. We infer comprised of approximately equal thicknesses of non- fluvial erosion because of the smooth, planar nature of the consolidated poorly sorted clastic deposits (e.g., sands, sub- erosion surface, the presence of sub-rounded clastic sediment rounded gravels, and coarse-grained, and angular rock ava- immediately above it, and the continued presence of the lanche deposits) intercalated with 2- to 5-m thick Lillooet River. The disconformity represents an unconstrained volcaniclastic deposits (e.g., welded and non-welded BAF time period that is missing from the rock record. This surface deposits). Unit B was previously included as part of the suggests a transient competition between dam building and overlying non-welded BAF deposit (unit C), implying the rise of the impounded lake wherein the dam was overtopped Bull Volcanol (2014) 76:811 Page 7 of 16, 811 by the rising lake at least once and perhaps periodically. This Lillooet River forms a >20 m wide-braided channel probably corresponded to a period of reduced volcanic flux; (Fig. 2b). There are three distinct sedimentary features subsequent waxing of volcanism renewed positive dam identified here: (1) a ∼0.75 km2 perched delta at the growth. confluence of the Salal Creek and the Lillooet River Erosion of unit A was followed by deposition of sand and (Fig. 5a); (2) a poorly exposed sequence of silts, sands, gravel, followed by multiple smaller volcanic events produc- and gravels forming the valley -floor; and (3) a pumice ing welded and non-welded BAF deposits and rock ava- and silt strandline deposit at 740 m asl. These sediments lanches. Our interpretation is hampered by our inability to are paracontemporaneous; they all contain PCF pumice sample the individual depositional units in unit B; however, clasts, are deposited on the same pre-PCF erosion sur- with the observations that we do have, we suggest that the face, and they are spatially restricted to the upstream clastic deposits situated immediately above the pronounced side of the BAF dam where a large lake would have erosional surface were deposited by lake water that been impounded. overtopped and eroded the dam, and remobilized pre- The thickest accumulations and coarsest sediments are existing valley–marginal sediments and volcaniclastic mate- observed in the delta at the outlet of Salal Creek (Cordey rials produced earlier in the eruption. The absence of thick 1999;Figs.2a, b and 5a). The top of the delta is at 740 m gravels at a similar elevation upstream of the dam suggests asl (Fig. 4) where it is ≥60-m thick. The lowermost exposed that the gravels were sourced from close to their current sediments comprise 1-cm thick off-lapping foreset beds of position and not from far upstream. Volcanism continued, rounded pumice clasts grading upwards into more heterolithic albeit less voluminously than before (unit A) or later (unit deposits (Fig. 5b). Coarse-grained foreset beds transition C), interspersed with rock avalanches from the southwest side sharply into flat-lying topset beds of low-angle cross-bedded of the Lillooet River valley. Regardless of the exact nature of sands (Fig. 5c). the sedimentary deposits, their presence, together with the Another pumice-rich deposit occurs as 1- to 2-m thick erosion surface, strongly indicate a period of water-induced lenses perched at the 740 m asl contour. The deposit com- denudation and sediment transport on top of the remains of the prises 20-cm-thick beds of rounded pumice clasts in a matrix welded BAF deposit dam, before waxing volcanism resumed of fine sand with silt and sand lenses, overlain by clast- dam construction. supported beds of rounded pumice lapilli of ≤4cmindiameter (Fig. 5d). We infer that these deposits were all formed in a Non-welded block and ash flow deposit—unit C standing body of water, paleo-Salal Lake, impounded upstream of the volcaniclastic dam. Rafts of pumice floating Unit C is a ∼50-m thick non-welded and locally incipiently on the surface of the lake were beached and reworked into welded BAF deposit (bx3 of Hickson et al. 1999) that is 1-m high berms by wave action defining a strandline at 740 m exposed in the upper walls of the Lillooet River canyon at asl and marking the lake’s maximum elevation (cf. White et al. elevations of 730 and 780 m asl (Fig. 3a) for <1 km down- 2001). stream of, Keyhole Falls (Fig. 4). The contact between unit B and the underlying sediment lens is sharp (Fig 3c). Unit C is characterized by crude, 2- to 5-m thick bedding and angular, Downstream volcaniclastic debris flow (lahar) deposits vesicular blocks in a matrix of pumiceous ash and is interpreted as BAF deposits for the same reasons as unit A (see above). A 3- to 10-m thick volcaniclastic deposit is exposed downstream of the mouth of the Lillooet River canyon (Fig. 2a)and Coeval epiclastic deposits for 3–4kmdownstreamofKeyholeFalls(Fig.3;sectionsF and G). It is characterized by abundant (∼40–60 vol.%) 0.5- We have identified several distinct sedimentary lithofacies that cm to 10-m diameter sub-angular to sub-rounded lapilli and parallel deposition of the BAF deposits (Fig. 4). These include blocks of the welded BAF deposit (unit A) in an unconsoli- (1) alluvial and lacustrine deposits comprising pumice-rich dated poorly sorted, fine-grained matrix of vitric shards, sand sands and gravels upstream of Keyhole Falls that partly onlap grains, and loose crystals. Many blocks feature radial prismat- the upstream margin of unit A and (2) a polymict, volcaniclastic ic joints (Fig. 6a) that get smaller towards the clast rim debris flow deposit (i.e., lahar deposit) downstream of the BAF (Fig. 6b). Many of the largest blocks are imbricated indicating deposits containing distinctive clasts of unit A. transport downstream. This deposit is interpreted as a debris flow (lahar) deposit (unit la; Hickson et al. 1999). The lahar Upstream fluvial and lacustrine deposits deposit thins and fines rapidly downstream of section G (Fig. 4) and is correlated with polymict, sand-rich In the Upstream of Keyhole Falls (Fig. 2a), the valley volcaniclastic layers 40–65 km downstream (Friele et al. bottom is uncharacteristically wide and flat, and the 2005; 2008). 811, Page 8 of 16 Bull Volcanol (2014) 76:811 a 685 m asl 0 soil d fine sand

Salal Creek

clast-supported, rounded Salal Creek delta pumice lapilli (< 4 cm), gravel Lillooet River

0.5 silt and fine sand with rounded pumice lapilli (< 2 cm)

clast-supported, rounded pumice lapilli (< 4 cm), gravel b c ash and sand with < 1 cm rounded pumice lapilli horizons 1 clast-supported, rounded pumice lapilli (< 4 cm), gravel fluvial gravel

beige sand matrix-supported, rounded pumice lapilli (< 4 cm), gravel - layers of silt and fine sand

beige silt - no pumice lapilli 1.5 m rounded pumice beige sand matrix-supported, clasts rounded pumice lapilli (< 4 cm), gravel - layers of silt and fine sand

base not seen Fig. 5 Deposits of paleo-Salal Lake. a Fan-shaped geometry of residual bedded and cross-bedded sands and gravels. d Graphic log of the upper delta at the confluence of Salal Creek and the Lillooet River. b Eroded 1.5 m of top-set beds, depicting alternating layers of sand and rounded cross-sections through the delta reveal thick beds of rounded pumice, 2,360 BP pumice (after Cordey 1999) interbedded with poorly sorted heterolithic conglomerate and c overlying

Discussion and modeling 1. The initial eruption was sub-Plinian and mantled the landscape with pumice (e.g., Hickson et al. 1999). Reconstruction of the volcanic dam and paleo-Salal Lake 2. Activity waned to Vulcanian explosions, emplacing hot (>Tg), BAF deposits into the Lillooet River valley imme- We summarize the 2,360 BP volcanic dam-building events diately below the vent that welded in situ (unit A); these (see Figs. 7 and 8)below. deposits ultimately accumulated to a total height of 110–

Fig. 6 The outburst flood a b deposit. a Boulder of welded BAF flow deposit within the poorly sorted lahar deposit at prismatic location F (Fig. 4 inset) joints characterized by well-developed prismatic joints on the clast margin. Person for scale. b Close- up of centimeter-scale prismatic jointing around the edge of a clast of welded BAF flow deposit within the debris flow deposit. Pencil for scale Bull Volcanol (2014) 76:811 Page 9 of 16, 811

a block and ash flow deposits avalanche 1000 lava basement deposits X unwelded welded X’

800

masl 740 m asl 680 m asl

600

0 500 1000 1500 2000 distance (meters along section)

460000 465000 470000 b 01234 Salal Creek 2000 km X’

lakeshore - 740 m asl

5620000 1000 1000 ke l La paleo-Sala X

5618000

5610000 5612000

1000 1000 2000

455000 460000 465000 100 1175 Canadian Cordillera (n = 21) d 3 1m/s 16 3 5m/s 12 3 750 top of dam (740 m) 39 d 51 d m/s d 98 546

e worldwide (n = 73)

l i 65 d aevlm 1 ) m lake volume (10

)

( t 700 283

h 14 d 19 d top of spillway (680 m)

50 e 24 d

141

39 65 e

l 650 63

l s

l 55

% 600 base level of c Lillooet River (580 m) 0 0 0 100 200 300 400 020406080 age of dam at failure (days) filling time (days) Fig. 7 Reconstruction and modeling of paleo-Salal Lake. a Valley-wide (dashed line) at 740 m asl. c The average longevity of landslide dams cross-section X–X′ (see b) through the dam at Keyhole Falls showing (see text) and the range of likely durations of the Keyhole Falls dam elevations of welded (unit A) and non-welded (unit B) deposits. b (dashed lines). d Model lake level elevations (m asl) and volumes of Topography of the upper Lillooet River valley, the distribution of the water for paleo-Salal Lake vs. time (days) based on maximum, mean, and Pebble Creek Formation (gray), and high-stand of paleo-Salal Lake minimum flow rates of the Lillooet River

170 m (Fig. 7a) and dammed the Lillooet River to form a and thin, discreet welded and non-welded BAF deposits lake (Fig. 7b). (unit B). 3. Lake level increased continuously but lagged behind the 5. Volcanism resumed producing a new sequence of cooler rising height of the dam (Fig. 8). Volcaniclastic sediment BAF deposits that generally did not weld (unit C). The washed into the lake. dam height increased by 60–100 m due to addition of this 4. Explosive volcanism waned, probably in response to ef- material (Fig. 7a) exceeding the rate of lake level rise fusion of cooler, less pressurized magma (Michol et al. (Fig. 8). The lake level increased gradually to 740 m asl 2008), while lake level continued to increase. The lake accompanied by progradation of the Salal Creek delta and overtopped the dam episodically and the weakly indurat- deposition of lacustrine deposits. ed upper portions of the dam were denuded to expose the 6. Volcanism waned such that the rise in dam height (addi- welded facies of the BAF (Fig. 8). Thin lenses of fluvial tion of volcaniclastic material minus viscous compaction) deposits accumulated on the erosion surface followed by a lagged behind the rate of lake level rise (Fig. 8). The dam thicker succession of intercalated rock avalanche deposits failed catastrophically when the lake level reached 740 m 811, Page 10 of 16 Bull Volcanol (2014) 76:811

Fig. 8 Time versus elevation

diagram depicting the growth i

a histories of the volcanic dam and t denudation; minor fluvial

l ‘hot’ BAF

u o and rockfall deposition

e l ‘cold’ BAF

f paleo-Salal Lake leading to the v deposits deposits final outburst flood event. The 810 240 relative volcanic flux is shown paleo-SL schematically 30 m t pumice d h

e g

t i 1 week i e strandline

s h 750 t o m t 740 m asl 180 h p a h ig e d ig e h

e h h e e B k

+100 m

a l i

d m

a h

d -60 m

l spillway

(

- 680 m asl m

690 w 120

)

& spillway

e elevation (m asl) t preserved

o t

+170 m h

e g m i d -1

630 e 0 60

1 A h WELDED CORE

e ~ n k m u a l excavation of U-shaped canyon 570 0 0 7 14 21 unknown duration 0 7 14 21 28 35 time (days)

asl: the high-stand elevation marked by pumiceous strand- Durations of dam building and lake filling line deposits. A canyon was incised through the dam into its welded core producing a lahar deposit containing “hot” Most natural dams fail only days after their formation: 62 % of clasts. landslide dams worldwide (n=73; Costa and Schuster 1988) 7. Paleo-Salal Lake drained to a low-stand elevation of and 85 % of the historical, landslide dams in the Canadian 680 m asl leaving a spillway floored by the resistant Cordillera (n=21; Clague and Evans 1994)failedwithin surface of the top of unit A (Fig. 7a). 50 days of formation, respectively (Fig. 7c). Only 10–15 % 8. Two thousand three hundred sixty years of Lillooet River of landslide dams impound semipermanent lakes for >1 year erosion formed the 60 m deep slot canyon (Costa and Schuster 1988; Clague and Evans 1994)priorto being passively overtopped or infiltrated. Overtopping is responsible for 90 % of dam failures (Costa and Schuster 1988); typically, erosion of the downstream edge of the dam forms a Timescales of dam building and lake filling knickpoint that rapidly incises back upstream through the dam. Here, we use the estimated dimensions and volumes of the We use the minimum high stand of the lake (740 m asl), its volcanic dam and paleo-Salal Lake to model the timescale of volume (546×106 m3), and its filling history to elucidate the lake filling and create a “water clock” for the building and construction of the dam and its ultimate failure. We used duration of the dam. We also model the size and duration of modern flow rates (mean minimum, 98 m3/s; monthly aver- the outburst flood produced during failure of the dam and aged, 125 m3/s; and mean maximum, 161 m3/s) of the Lillooet estimate the duration of 2,360 BP eruption. River measured over a 96 years period (Water Survey of Canada 2012) to compute the time needed to fill the paleo- lake. In all cases, we assume zero leakage through the dam; Dam and paleo-lake dimensions low dam permeability is justified by the low porosity (ϕ< 10 %; Michol et al. 2008) and low crack density of the welded The volcanic dam had an asymmetrical, bivergent, tapering BAF deposits (unit A). wedge-like geometry (Fig. 4 inset; Michol et al. 2008), a Based on these flow rates, the lake filled to 680 m asl in 14– minimum height greater than 160 m (Table 1), and a volume 24 days at which point it reached the top of unit A (the welded of ∼1.5×108 m3 (Table 1). The volume of paleo-Salal Lake is core of the dam; Figs. 7d and 8). Then, for an unconstrained based on the maximum lake level elevation (740 m asl; period of time (probably days to weeks), the lake episodically Fig. 7a), and the pre-2,360 BP topography of the upstream overtopped, eroded, and redeposited the upper surface of the Lillooet River valley assuming a pre-eruption bedrock surface dam and adjacent non-consolidated deposits (unit B). As at ∼580 m asl. Adopting a V-shaped geometry similar to the volcanism renewed, the dam built up another 60–100 m (unit downstream (i.e., non-sedimented) valley, the GIS-estimated C); the second lake-filling event took an additional 25 to volume for the paleo-Salal Lake is 546×106 m3, the surface 41 days to fill up to 740 m asl. Total lake-filling time was area is 11.3×106 m2, and the depth is 160 m prior to dam between 39 and 65 days excluding the unconstrained time failure and ∼100 m after dam failure (i.e., Fig. 7a). period (Fig. 8); a review of natural dam durations shows that Bull Volcanol (2014) 76:811 Page 11 of 16, 811 the Pebble Creek Formation dam is in the 60–80th percentile 3. post-dam failure (present day) range worldwide (Costa and Schuster 1988;Fig.7c). Our estimate of 39–65 days for the minimum duration of unit A unit B the 2,360 BP eruption (Fig. 8) compares well with the durations of VEI 4 or greater eruptions between 1600 and 2010 A.D. compiled in the Global Volcanism Program database 2. continued deposition and welding of BAF deposit; intermittent fluvial erosion and deposition; (Siebert and Simkin 2002;Appendix1). The median duration intermittent rock avalanches of all VEI 4+ eruption episodes (n=129) is 55 days; the 40% compaction 20% compaction median duration of VEI 4 eruptions from stratovolcanoes specifically (n=78) is 47 days.

1000

)

Paleoviscometry: competition between dam growth l

s 1. BAF deposition and compaction a

(

800

Field evidence suggests that the thick succession of BAF v

l 600 deposits resulted from accumulation of many smaller- e volume pyroclastic density currents into a narrow, confined 0 500 1000 1500 2000 river valley (Hickson et al. 1999); the accumulation was dam width (m) block and ash flow deposits sufficiently rapid to keep the deposits above their Tg lava (>500 °C) allowing the deposits to weld as a single cooling non-welded welded fluvial avalanche basement unit. The unconsolidated deposits have an average matrix deposits deposits porosity of ∼41 % (38–45 %) and vitric clasts have an average ∼ – Fig. 9 Reconstructed sequential paleo-cross-sections for Keyhole Falls porosity of 32 % (24 40 %; cf. Michol et al. 2008). The – ε pyroclastic dam. 1 Reconstructed cross-section prior to 30 40 % short- reduction in porosity within unit A implies a total strain ( T)of ening due to compaction and welding (Michol et al. 2008). 2 Recon- ∼38 % due to compaction (volume strain; Michol et al. 2008). structed geometry of pyroclastic deposits immediately prior to failure and The average minimum integrated strain based on compaction after compaction and welding (see text). Dashed lines are inferred iso- compaction lines for 20 and 40 %. 3 Present-day geometry of residual alone (ignoring shear strain) for the entire vertical section of ∼ ∼ pyroclastic ( 110-m thick) deposits and canyon. Dashed lines are inter- the deposit is 31 %, implying >50 m shortening of the polated contacts across the canyon thickest deposit (from an original ∼160 m to the current ∼110 m; Fig. 9). Below, we use the extent of compaction and the time paleo-viscometry study of welding of pyroclastic successions required to fill the lake prior to dam failure to compute an wherein field evidence constrains the viscosity; similar studies effective bulk viscosity (η). We assume incremental accumu- on other rock types are uncommon (e.g., Höink et al. 2008) lation and compaction of the pyroclastic deposits that just and reliable methods of paleo-viscosity estimation remain balances the lake-filling time (Fig. 10a). This incremental elusive (Talbot 1999). model for welding and compaction simulates the progressive We suggest that 1010 Pa s is a valid estimate for the bulk deposition of unit A from a series of discreet block and ash viscosity of these dacitic pyroclastic deposits and that these flows produced by small-volume Vulcanian explosions (e.g., results meet the minimum conditions for welding (e.g., Michol et al. 2008). We have used an intermediate lake filling Russell and Quane 2005). We calculated a 1010 Pa s isokom rate to set the accumulation rate of the deposits prior to the for melt viscosity (i.e., Giordano et al. 2008) assuming a melt lake first overtopping the dam at ∼110 m (e.g., 14–24 days). composition equivalent to the vitric blocks (Stewart 2002; Material accumulates in 10 m increments generating a load Fig. 10b). The 1010 Pa s isokom is consistent with emplace- : stress (σ)of∼0.03 MPa, and the strain rate (ε ) is taken as the ment temperatures between 795 and 600 °C depending on total compaction strain (31 %) divided by the incremental water content (Fig. 10b). Stewart (2002) measured residual (10 m) filling time. The viscosity over the increment is calcu- water contents in the glassy pyroclasts within the BAF de- lated from: posits of 0.3–1.1 wt% (mean (n=13) ∼0.7 wt%) suggesting : accumulation temperatures of 660–795 °C. The Tg marks η ¼ σ=ε ð1Þ where welding processes cease and can be computed as the temperature where η ∼1012 Pa s. This estimate of Tg (Fig. 10b) lies ∼80 °C below the emplacement temperatures implied by The deposits have a model η of between 109.5 (early) to our field-constrained viscosity values (Fig. 10a). Slow con- 1010.2 Pa s (later) (Fig. 10a). We anticipate this as the first ductive cooling of the deposits could exploit this +80 °C 811, Page 12 of 16 Bull Volcanol (2014) 76:811

a difference to provide the time required for compaction and 13 welding. The deposit was still above its Tg when the dam

12 failure occurred as indicated by the radially jointed blocks within the lahar deposit. However, there is no evidence of 11 rheomorphic (i.e., shear stress driven) flow of the welded BAF deposit into the newly excavated canyon (c.f. Kamata et al.

)

s . 10 1993).

P ( Figure 10c summarizes the rates and timescales of dam- 9 incremental model

10 and lake-building events recorded by the PCF, including (1)

o l construction of the pyroclastic dam, (2) filling of paleo-Salal 8 Lake behind the dam, and (3) failure of the dam. The rise in 7 lake level with time (heavy black line) is limited by the height of the dam; the rate of lake rise (slope) depends on the average 14 d 19 d 24 d 6 discharge rates of the Lillooet River, the geometry of the 0 5 10 15 20 25 30 filling time (days) upstream river valley, and the integrity of the dam. The rate b at which the dam height increases with time depends on the 900 rate of accumulation of pyroclastic material within the Lillooet

H2 O < 0.7 wt.% valley, the rate of compaction, and the rate of removal by streams if the lake overtops the dam. The deposition of pyro- 800 clastic material can be near instantaneous (curve 1), smoothly decreasing (curve 2), or episodic (curve 3), depending on the

) style of volcanism. Compaction reduces the height of the dam

( 700 and can occur during or after accumulation. The accumulation 10 T ~10 Pa.s curves are initially higher than the lake-filling curve and the lake level is free to rise against the increasing height of the 600 Tg pyroclastic dam. As rates of volcanic eruption diminish, the lake level rises to meet the top of the dam (after ca. 39–65 days at Mount Meager). At this point, the dam failed and the lake

500 drained rapidly (Fig. 10c; note the change in relative 0 0.5 1 1.5 timescale). wt. % H2 O c Dam failure and duration of volcanism max. pre-compaction height

1 The outburst flood compactiondeposition Thick debris flow deposits downstream of Keyhole Falls attest height at failure to a massive outburst of sediment-laden water (i.e., a lahar)

t resulting from the collapse of the dam. Prismatic and chaotic

g i 2

e jointing on the faces of the cliff walls in unit A (Fig. 3a) h g 3 lin indicates that the deposit was hot when the dam failed. fil l - ve Another indication that the core of the dam was at high le e k la

la t k temperature at failure are the delicate prismatic joints found

n e

o d

p ra on blocks of unit A (Fig. 6) within the lahar. The hot outburst

e in r i

u n

i g flood deposits indicate the canyon-forming failure that oc-

a f curred long enough after eruption to allow the deposits to weeks / days hours weld completely and develop coarse vertical cooling columns Fig. 10 a Values of log η, based on incremental (10 m) accumulations of (e.g., Fig. 3c) but short enough to prevent the dam from material and a strain rate proportional to the incremental filling time. b cooling below Tg. The 1010 Pa s isokom for vitric blocks within unit A and the glass We have developed an iterative model for the deluge η ∼ 12 transition temperature (Tg) for the melt (i.e., 10 Pa s; Giordano caused by the dam failure and responsible for carving out et al. 2008) as a function of temperature and water content. c Schematic models for the rise in dam height (see text) lines 1, 2, and 3, with the canyon in front of Keyhole falls. Our dam failure model comparison to changes in lake level. Dam growth is curtailed when the is for drainage through a time-varying breach and uses a lake level exceeds the height of the dam standard broad-crested weir equation for a trapezoidal- Bull Volcanol (2014) 76:811 Page 13 of 16, 811

Ta b l e 2 Summary of parameters used to model dam failure and resulting where the drag coefficient (Cd) is taken as 0.57 for a trape- outburst flood zoidal breach, W is the base width of the trapezoidal breach, θ Elevation Vo lu me Fill timea and is the side wall angle (Manville 2001). The equation (m asl) (×106 m3) (days) produces a conservative prediction because it does not account for frictional and turbulent flow resistance in the outlet Base of dam 580 channel. We allow for incremental growth of the breach over Spillway lake 680 200 14.4–23.7 the first 2 h to attain a final geometry of 250 m for the top Peak lake 740 546 39.3–64.6 width, 100 m for the base width, and a depth of 60 m (e.g., Released water 346 vertical erosion rate through unconsolidated dam of 0.5 m/ Sediments entrainedb 5.25 min). Our iterative model for the deluge employs the follow- Post-failure canyon 580–500 45 ing time incremented (1–100 s) steps: a Minimum and maximum times based on seasonal flow rates (see text) 1. Compute the volume of the lake to be released during b Unconsolidated sediments entrained during dam collapse (bulk factor is failure (i.e., V=346×106 m3; Table 2;Fig.11a). 1.5 % by volume) 2. Compute the current base width (W) and side wall angle (θ) of the trapezoidal breach and the head over the base of shaped breach (e.g., Walder and O’Connor 1997; Fenton et al. the weir (H) assuming continuous progressive growth 2006). Specifically, we have adopted the formulation of over the first 2 h. Manville (2001) which relates discharge (Q) through the 3. Compute Q (Eq. 2). breach to the pressure head over the weir (H): 4. For a fixed time interval (1–100 s), use Q to predict the amount of water to escape and thereby reduce the lake ¼ : 1:5½þ ðÞθ ðÞ Q 0 591CdH 5W 4tan H 2 volume infinitesimally (e.g., drain; Fig. 11b).

600 60 740 550 730 500 63 40 volume of

)

) sediment

3 450 released ( 720

20 e

k released (10 m )

l 400 volume of sediment

( 710

m 350 0 h

u 0 4 8 12 16 20 g

l i 700

o time from failure (hours) e

v 300 h volume head water & sediment 690 250 346 x1063 m water only 680 200 lake volume below spillway a spillway height b 150 670 0 2 4 6 8 10 0 2 4 6 8 10 time from failure (hours) time from failure (hours) 0 c 12 5 velocity of water 10 500 instantaneous erosion rate height 8

) 1000 reduction

/ of loose distance carved from front of dam

( 1500 dam

4 4 e velocity (m/s) 10 t

)

- 2000

3 Keyhole Falls

)

m 0 m 2500

( 0 4 8 12 16 20 (

time from failure (hours) c

3 a 3000 10 b ambient flow t

u after ~20 hrs c 3500 breaching phase of mean flow rate 4000 loose of Lillooet River sediment dam d 102 4500 0 4 8 12 16 20 0246810 time from failure (hours) time from failure (hours) Fig. 11 Model parameters calculated (see text) for outburst flood (cf. flood water and debris (dashed line) with time. d Model values for canyon Fig. 3). a Volume of water behind dam from failure to end of flood (∼4h). cut back (m) and the instantaneous rates of canyon cut back (m/h) plotted Dashed line indicates effects of bulking up of flood water by sediments at vs. time (h). Cutback ends immediately before Keyhole Falls, thereby dam head and by debris from canyon excavation. b Model decrease in preserving a fraction of the dam lake level as a function of time during flood. c Calculated discharge of 811, Page 14 of 16 Bull Volcanol (2014) 76:811

108 5. Recompute the new lake volume and repeat until the lake Icelandic drains to the final spillway elevation (680 m asl) and the 107 jökulhlaups 2360 BP Pebble flux is equal to the discharge rate of Lillooet River 6 Creek formation volcanic 10 LGM

)

(Fig. 11c). / deglaciation

3 5

( 10

We allowed the escaping water to entrain unconsolidated, a 4

h 10 landslide

s water-saturated sediment from the top of the dam (unit B). The i

d 3 k 10 entrainment is implemented incrementally and lead to a a man-made dams e moraine ∼ p bulking-up of the flood waters by 1.5 vol.% (Fig. 11a). 102 This high velocity flow of sediment-laden water provides 101 the momentum for eroding the downstream canyon. Erosion glacial starts at the toe where the dam is <10-m thick and eventually 100 incises its way back to Keyhole Falls. The total canyon 10-3 10-2 10-1 100 101 102 103 104 105 106 107 63 volume is 45×106 m3;thismaterialmustbeentrainedinthe clearwater flood volume (x 10 m ) flood water and leads to an additional 13 vol.% bulking of the Fig. 12 Volume and peak discharge rates for volcanic, glacial, landslide, moraine, and other outburst flood types (from Kataoka et al. 2008) and for flood (Fig. 11a). The grade over 8.7 km below the current this work (star) Keyhole Falls to the confluence with Meager Creek is 7.5 % suggesting that this level of bulking is physically reasonable (c.f. Pierson 2005b). Substantially higher bulking factors ∼3 Conclusions (e.g., 2:1 sediment–water) can create high-concentration sediment–water mixtures or debris flows having the consistency The VEI 4 eruption of Mount Meager 2,360 BP produced a of wet concrete that move as “plug flows” and can entrain car- short-lived volcanic dam that impounded a lake, and eventu- sized blocks of rock (e.g., Pierson 2005a, b). At Mount ally failed, thereby, releasing an outburst flood. Dacitic BAF Meager, blocks of the dam material in excess of 15 m in deposits rapidly accumulated in an apron close to the vent, and diameter have been transported only hundreds of meters from welded in the center where thickest and most insulated. The their source before being deposited; this probably indicates impounded lake level rose as volcanism waned to a point that the lower gradient (7.5 %) did not allow the flood to where water spilt across the top of the dam, eroding and entrain more than 15–20 vol.% sediment (e.g., bulk factor reworking the non- and less-welded upper parts. Volcanism <1.5). increased again restoring and growing the dam before it Our model for overtopping of the dam and the asso- catastrophically failed and released an outburst flood as the ciated breakout flood (Fig. 11) shows the main deluge lake drained. The outburst flood eroded through the dam to the lasting for ∼8 h as the lake level drops from 740 to welded core, and produced a lahar deposit that included clasts 680 m asl, before waning to background discharge rates of still-hot welded BAF deposit. Positive dam building oc- within ∼20 h (Fig. 11b, c). The peak discharge rate curred over 1–2 months, based on calculated rates of lake (2.5 h after failure) is 5.4×104 m3/s (water+sediments) filling, during which time the core was able to weld and which delivers the extreme erosive potential (Clague and compact by 31 %, with an effective viscosity of 1010 Pa s. Evans 2000) needed to carve the canyon and transport The outburst flood lasted ∼20 h and released ∼346×106 m3 of ∼15-m diameter boulders. This peak discharge is similar water and additional eroded dam material downstream, com- to, or larger than, many other recorded and reconstructed parable with other volcanic dam failure events. volcanic-, landslide-, or ice-dammed outbursts (Fig. 12). This study highlights the complex and sustained develop- The decreasing flux is due to the constantly decreasing ment of an otherwise typical stratovolcano eruption, and the hydraulic head behind the dam (Fig. 11a). evolving nature of its associated deposits and geological haz- Figure 11d shows the instantaneous erosion rate (m/h) and ards. Uniquely, for a pre-historic event, we demonstrate the the position of the knickpoint relative to the original front of duration of the eruption and the different timescales of dam the dam as a function of time. Peak longitudinal erosion rates building, welding, dam failure, and outburst flooding. at the start of the flood are several kilometers per hour but rapidly decrease to several hundred meters per hour, and fall off exponentially after that (Fig. 11d). Similarly, the position Acknowledgments Research costs were met through support from of the knickpoint rapidly moves upstream for ∼1.5 h. After NSF CREST award 1137774 and a CSU Bakersfield RCU award to 2,000 m of longitudinal erosion, the knickpoint has reached its GDMA, an NSERC Discovery Grant to JKR, and a Postgraduate (PGS- final position at Keyhole Falls (Fig. 3a); had the lake volume A) scholarship to MS. Thoughtful reviews and comments from Vernon Manville, Nancy Riggs, Brittany Brand, Adrian Pittari, and an anony- been greater and the flood sustained longer, the dam breaching mous reviewer greatly improved the clarity of our arguments and the would have been complete. manuscript in general. Bull Volcanol (2014) 76:811 Page 15 of 16, 811

Appendix

Fig. 13 Histogram of the measured or estimated durations of VEI 4 or greater eruptions from stratovolcanoes and calderas between 1600 A.D. and 2010, complied from the Global Volcanism Program database (Siebert and Simkin 2002)

References Costa JE, Schuster RL (1988) The formation and failure of natural dams. Geol Soc Am Bull 100:1054–1068 Fenton CR, Webb RH, Cerling TE (2006) Peak discharge of a Pleistocene Andrews GDM, Russell JK, Brown SR, Enkin RJ (2012) Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA. Quat Res reversal of the Fraser River, British Columbia. Geology 40:111–114 65:324–335 Auker MR, Sparks RSJ, Siebert L, Crosweller HS, Ewert J (2013) A Friele PA, Clague JJ, Simpson K, Stasiuk MV (2005) Impact of a statistical analysis of the global historical volcanic fatalities record. J Quaternary volcano on Holocene sedimentation in Lillooet River Appl Volcanol 2:2. doi:10.1186/2191-5040-2-2 valley, British Columbia. Sediment Geol 176:305–322 Capra L (2007) Volcanic natural dams: identification, stability, and sec- Friele PA, Jakob M, Clague J (2008) Hazard and risk from large land- ondary effects. Nat Hazards 43:45–61 slides from Mount Meager volcano, British Columbia, Canada. Capra L, Macías JL (2002) The cohesive Naranjo debris-flow deposit Georisk Assess Manag Risk Eng Syst Geohazards 2:48–64 (10 km3): a dam breakout flow derived from the Pleistocene debris- Giordano D, Russell JK, Dingwell DB (2008) Viscosity of magmatic avalanche deposit of Nevado de Colima volcano (Mexico). J liquids: a model. Earth Planet Sci Lett 271:123–134 Volcanol Geotherm Res 117:213–235 Hickson CJ, Russell JK, Stasiuk MV (1999) Volcanology of the Carter CL, Anderson RS (2006) Fluvial erosion of physically modeled 2350 B.P. eruption of Mount Meager Volcanic Complex, abrasion-dominated slot canyons. Geomorphology 81:89–113 British Columbia, Canada: implications for hazards from Clague JJ, Evans SG (1994) Formation and failure of natural dams in the eruptions in topographically complex terrain. Bull Volcanol Canadian Cordillera. Geol Surv Can Bull 464:1–35 60:489–507 Clague JJ, Evans SG (2000) A review of catastrophic drainage of Hodgson KA, Nairn IA (2005) The AD 1315 syn-eruption and AD 1904 moraine-dammed lakes in British Columbia. Quat Sci Rev 19: post-eruption breakout floods from Lake Tarawera, Haroharo calde- 1763–1783 ra, North Island, New Zealand. NZ J Geol Geophys 48:491–506 Clague JJ, Harper JR, Hebda RJ, Howes DE (1982) Late quaternary sea Höink T, Lee CA, Hawthorne JC, Lenardic A (2008) Paleo-viscometry of levels and crustal movements, coastal British Columbia. Can J Ear magma bodies. Earth Planet Sci Lett 267:100–106 Sci 19:597–618 Kamata H, Suzuki-Kamata K, Bacon CR (1993) Deformation of the Cordey P (1999) Sedimentological evidence for the damming of the Wineglass Welded Tuff and the timing of caldera collapse at Lillooet River by the 2350 B.P. eruption of Mount Meager, Crater Lake, Oregon. J Volcanol Geotherm Res 56:253–265 Southern Coast Mountains, B.C. BSc thesis, University of British Kataoka KS, Urabe A, Manville V, Kajiyama A (2008) Breakout Columbia, Vancouver flood from an ignimbrite-dammed valley after the 5 ka 811, Page 16 of 16 Bull Volcanol (2014) 76:811

Numazawako eruption, northeast Japan. Geol Soc Am Bull Russell JK, Quane S (2005) Rheology of welding: inversion of field 120:1233–1247 constraints. J Volcanol Geotherm Res 142:173–191 Macías JL, Capra L, Scott KM, Espíndola JM, García-Palomo A, Costa Siebert L, Simkin T (2002) Volcanoes of the World: an illustrated JE (2004) The 26 May 1982 breakout flows derived from failure of a catalog of Holocene volcanoes and their eruptions. volcanic dam at El Chichón, Chiapas, Mexico. Geol Soc Am Bull Smithsonian Institution, Global Volcanism Program Digital 116:233–246 Information Series, GVP-3 (http://www.volcano.si.edu/ Manville V (2001) Techniques for evaluating the size of potential dam- world/). Accessed 17 Feb 17 2012 break floods from natural dams. Inst Geol Nuc Sci 144 p Simpson KA, Stasiuk MV, Shimamura K, Clague JJ, Friele P (2006) Manville V, Cronin SJ (2007) Breakout lahar from New Zealand’s Crater Evidence for catastrophic debris flows in Pemberton Valley, British Lake. Eos Trans AGU 88:441–442. doi:10.1029/2007EO430001 Columbia. Can J Ear Sci 43:679–689 Manville V, Wilson CJN (2004) The 26.5 ka Oruanui eruption, New Smith RL (1960) Zones and zonal variations in welded ash flows. US Zealand: a review of the roles of volcanism and climate in the post- Geol Surv Prof Pap 354F:149–159 eruptive sedimentary response. NZ J Geol Geophys 47:525–547 Stasiuk MV, Russell JK, Hickson CJ (1996) Distribution, nature, and Manville VR, White JDL, Houghton BF, Wilson CJN (1999) origins of the 2400 B.P. eruption products of Mount Meager, British Paleohydrology and sedimentology of a post-1800a breakout flood Columbia: linkages between magma chemistry and eruption behav- from intracaldera Lake Taupo, North Island, New Zealand. Geol Soc iour. Geol Surv Can Bull 486:27 p Am Bull 111:1435–1447 Stewart ML (2002) Dacite block and ash avalanche hazards in mountain- Manville V, Hodgson KA, Nairn IA (2007) A review of break-out floods ous terrain: 2,360 yr. B.P. eruption of Mount Meager. MSc thesis, from volcanogenic lakes in New Zealand. NZ J Geol Geophys 52: University of British Columbia, Vancouver 131–150 Talbot CJ (1999) Can field data constrain rock viscosities? J Struct Geol Michol KA, Russell JK, Andrews GDM (2008) Welded block and ash 21:949–957 flow deposits from Mount Meager, British Columbia, Canada. J Walder JS, O’Connor JE (1997) Methods for predicting peak discharge of Volcanol Geotherm Res 169:121–144 floods caused by failure of natural and constructed earth dams. Pierson TC (2005a) Distinguishing between debris flows and floods from Water Resour Res 33:2337–2348 field evidence in small watersheds. US Geol Surv Factsheet 2004- Water Survey of Canada. Archived hydrometric data, Lillooet 3142:4 p River, British Columbia. http://www.wsc.ec.gc.ca/. Accessed Pierson TC (2005b) Hyperconcentrated flow—transitional process be- March 2012 tween water flow and debris flow. In: Jakob M, Hungr O (eds) White JDL, Manville VR, Wilson CJN, Houghton BF, Riggs N, Ort M Debris flow hazards and related phenomena. Springer, Berlin, pp (2001) Settling and deposition of 181 A.D. Taupo pumice in lacus- 159–201 trine and associated environments. In: White JDL, Riggs NR (eds) Read PB (1977) Meager Creek volcanic complex, southwestern British Volcanogenic sedimentation in lacustrine settings, vol 30, Int Ass Columbia. Rep Act Geol Surv Can Paper 77-1A:277–281 Sed Spec Publ., pp 151–177